On Wed, 25 May 2016, Kent Overstreet wrote: > On Wed, May 25, 2016 at 02:47:29PM -0700, Eric Wheeler wrote: > > > > I have a few bcachefs questions that pertain to its use with block > > devices. > > > > Does bcachefs's implementation reuse and update the existing > > bcache code such that the block device driver inherits the bcachefs > > improvements? I understand the cache superblock changed, maybe the cached > > dev super too. > > Yes, all of the existing functionality is still there (though some of it's > broken at the moment because I haven't been running those tests; if you're > interested in using bcache-dev for the old style caching (there are performance > and robustness improvements) it wouldn't take me long to get it working again). > > > Can bcachefs provide /dev/bcacheN devices without loop.ko? > > > > If so, are these simply filesystem objects (files)? > > No, at least not currently - the "export a block device" code and the filesystem > code are effectively thin wrappers around the core bcache IO path (bch_read() > and bch_write()) - but the two different interfaces don't have anything to do > with each other. > > The way it works is the first 4096 inode numbers are owned by the block device > interface - inodes in that range are for either cached devices or thin > provisioned volumes. The filesystem code owns inode numbers >= 4096. > > So while blockdev volumes/cached data do have inodes, they're not reachable via > the filesystem because there will never be dirents that point to them (also, > they use a different inode type with extra fields for the UUID/label). > > However - there isn't anything preventing us from writing a bit of new code and > hooking it up to an ioctl or sysfs interface to say "look up this path and > create a block device for that file". The only remotely tricky bit would be > pagecache cache coherency, but I think you get the new block device to just use > the same address_space (pagecache cache for an inode) as the filesystem inode. > > So, probably doable in ~100 lines of code or so. Hey Kent, I noticed that blockdev.c got an overhaul in bcache-dev recently. Getting closer to blockdev export from bcachefs? Actually it looks like you've been pretty busy on that branch in general. What's new? -- Eric Wheeler > -- > To unsubscribe from this list: send the line "unsubscribe linux-bcache" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- To unsubscribe from this list: send the line "unsubscribe linux-bcache" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
